kW to kVA: How to Size a Diesel Generator Without Guessing
By Tommy, Export Sales Manager at Zhiqiang Power · tommy@zhiqiang-power.com · Last reviewed 2026-09-17 against factory load-bank data
Most sizing mistakes we see are unit mistakes, not engineering mistakes: a buyer totals the kW on his equipment list, orders "a 100 kVA set" for a 100 kW load, and the generator arrives about 20% too small. This page gives you the conversion, the margin rules and the motor-start check in that order.
The short answer
For three-phase industrial generators the rated power factor is 0.8, so kVA = kW ÷ 0.8. A 100 kW requirement means a 125 kVA set, before you add any margin.
- ›kW is the real power your equipment does work with; kVA is the apparent power the generator must supply. At cos φ 0.8, 1 kVA delivers 0.8 kW.
- ›Add 20–25% reserve on top of your running load so the set sits in its efficient band and can absorb peaks without tripping.
- ›Direct-on-line motor starts pull 3–6× full-load current for a few seconds; the largest motor often decides the set size, not the total load.
- ›kW vs kVA also matters when comparing quotes: some suppliers lead with the bigger number. Always ask which rating the figure refers to.
The conversion and a quick table
The formula runs both ways: kVA = kW ÷ 0.8 and kW = kVA × 0.8. The table below covers the most requested sizes in our catalogue; each row links to the matching model page.
| Your load (kW) | Generator needed (kVA) | Typical catalogue model |
|---|---|---|
| 50 kW | 62.5 kVA | TFW-50 (50 kW / 62.5 kVA) |
| 100 kW | 125 kVA | TFW-100 (100 kW / 125 kVA) |
| 200 kW | 250 kVA | TFW-200 (200 kW / 250 kVA) |
| 300 kW | 375 kVA | TFW-300 (300 kW / 375 kVA) |
| 500 kW | 625 kVA | TFW-500 (500 kW / 625 kVA) |
| 800 kW | 1000 kVA | TFW-800 (800 kW / 1000 kVA) |
Model names in this table link to the corresponding product page, where the prime and standby ratings, fuel curve and dimensions are published.
Motor starting current: the number that really sizes the set
A motor started direct-on-line draws three to six times its rated current until it reaches speed. If your largest machine is a 30 kW pump on DOL starting, the generator has to swallow that inrush without a voltage dip deep enough to trip contactors. In practice this means checking the alternator datasheet for motor-starting capability, sometimes choosing a frame larger than the running kW suggests, or using star-delta or soft starting on the biggest motors.
How much margin should you add?
For standby duty, size against the essential load you will actually shed to, plus 20–25%. For prime or continuous duty, aim to sit between 50% and 80% of prime power during normal running: below that band the engine glazes, above it you are living on the overload allowance.
Send us the load list, the largest motor and its starting method, and the duty (standby, prime or continuous). We will return a sizing sheet showing the calculation, not just a model number.
Related reading
Power ratings follow ISO 8528-1:2018 (Prime PRP / Standby ESP), and the fuel figures are stated at ISO 3046-1:2002 standard reference conditions. Both are confirmed unit by unit on our load bank, and the test record ships with the set. Standby figures are the engine's factory standby rating from the model spectrum table — never a computed 110% of the rated output. Noise figures are typical open-type reference values, not a guaranteed sound-power level.
Frequently Asked Questions
Is 100 kW the same as 100 kVA?▼
No. At the standard 0.8 power factor, 100 kW needs a 125 kVA generator, and a 100 kVA set can only deliver 80 kW. If two suppliers quote different-looking numbers for the same machine, one is probably quoting kVA and the other kW.
What power factor do diesel generators use?▼
Industrial three-phase gensets are rated at 0.8 lagging, per the alternator nameplate and ISO 8528-1 practice. Single-phase loads should be balanced across the three phases; a badly unbalanced load derates the set and heats the alternator.
Should I buy a bigger generator “just in case”?▼
One size of headroom is sensible; two is not. A set that never passes 30% load runs wet, glazes its bores and needs early top-end maintenance. Give us the load list and growth plan and we will size for where you will actually be in two years.
Get a technical quotation for your project
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